Just update the array indices of the required elements

In my test case, I changed the indices used in the script. The script used would be as follows:

function myFunction() {
  var array = [
    [2, 'name1', 'something', 15],
    [3, 'name10', 'something', 5],
    [5, 'name20', 'something', 20],
    [2, 'name15', 'something', 3]
  ]
  var result = Object.values(array.reduce((c, v) => {
    if (c[v[0]]) c[v[0]][3] += v[3]; // Updated the indices
    else c[v[0]] = v; // Updated the indices
    return c;
  }, {}));

  console.log(result);
}

From here, index [0] represents the elements in the first column (2,3,5,2) while index [3] represents the elements in the last column (15,5,20,3). So basically, the script only processed the first and last columns to achieve your desired output.

Output